This invention relates to electro-optic displays and to adhesive compositions for use therein. More specifically, this invention relates to adhesive compositions with properties, including mechanical, electrical, and chemical properties, which render them especially suitable for use in electro-optic displays, and which facilitate the manufacture of such displays and increase the useful life thereof. The adhesive compositions of the present invention may also be useful in applications other than electro-optic displays.
Electro-optic displays comprise a layer of electro-optic material, a term which is used herein in its conventional meaning in the art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. The optical property is typically color perceptible to the human eye, but may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
The electro-optic displays of the present invention typically contain an electro-optic material which is a solid in the sense that the electro-optic material has solid external surfaces, although the material may, and often does, have internal liquid- or gas-filled spaces, and to methods for assembling displays using such an electro-optic material. Such displays using solid electro-optic materials may hereinafter for convenience be referred to as xe2x80x9csolid electro-optic displaysxe2x80x9d. Thus, the term xe2x80x9csolid electro-optic displaysxe2x80x9d includes encapsulated electrophoretic displays, microcell electrophoretic displays and encapsulated liquid crystal displays.
One type of electro-optic display is the rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782 and 5.760,761 (this type of electro-optic medium is often referred to as a xe2x80x9crotating bichromal ballxe2x80x9d medium, but the term xe2x80x9crotating bichromal memberxe2x80x9d is preferred since in some versions of the medium the rotating members are not spherical).
Another type of electro-optic medium is an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O""Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in International Applications Publication Nos. WO 98/35267 and WO 01/27690, and in copending applications Ser. Nos. 60/365,368; 60/365,369; 60/365,385 and 60/365,365, all filed Mar. 18, 2002, and applications Ser. Nos. 60/319,279; 60/319,280; and 60/319,281, all filed May 31, 2002; the entire contents of all these applications are herein incorporated by reference.
Another type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a suspending fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,241,921; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; and 6,413,790; U.S. patent applications Publication Nos. 2001-0045934; 2002-0018042; 2002-0019081; 2002-0021270; 2002-0053900; and 2002-0060321; and International Applications Publication Nos. WO 97/04398; WO 98/03896; WO 98/19208; WO 98/41898; WO 98/41899; WO 99/10767; WO 99/10768; WO 99/10769; WO 99/47970; WO 99/53371; WO 99/53373; WO 99/56171; WO 99/59101; WO 99/67678; WO 00/03349; WO 00/03291; WO 00/05704; WO 00/20921; WO 00/20922; WO 00/20923; WO 00/26761; WO 00/36465; WO 00/36560; WO 00/36666; WO 00/38000; WO 00/38001; WO 00/59625; WO 00/60410; WO 00/67110; WO 00/67327 WO 01/02899; WO 01/07691; WO 01/08241; WO 01/08242; WO 01/17029; WO 01/17040; WO 01/17041; WO 01/80287 and WO 02/07216. The entire disclosures of all these patents and published applications are herein incorporated by reference.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, WO 01/02899, at page 10, lines 6-19. See also copending application Ser. No. 09/683,903, filed Feb. 28, 2002, the entire disclosure of which is herein incorporated by reference, and the corresponding International Application PCT/US02/06393.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word xe2x80x9cprintingxe2x80x9d is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
A related type of electrophoretic display is a so-called xe2x80x9cmicrocell electrophoretic displayxe2x80x9d. In a microcell electrophoretic display, the charged particles and the suspending fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Applications Publication No. WO 02/01281, and published U.S. application Ser. No. 2002-0075556, both assigned to Sipix Imaging, Inc.
Other types of electro-optic materials, for example, polymer-dispersed liquid crystal, may also be used in the displays of the present invention.
In addition to the layer of electro-optic material, an electro-optic display normally comprises at least two other layers disposed on opposed sides of the electro-optic material, one of these two layers being an electrode layer. In most such displays both the layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongate row electrodes and the other into elongate column electrodes running at right angles to the row electrodes, the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display. In another type of electro-optic display, which is intended for use with a stylus, print head or similar movable electrode separate from the display, only one of the layers adjacent the electro-optic layer comprises an electrode, the layer on the opposed side of the electro-optic layer typically being a protective layer intended to prevent the movable electrode damaging the electro-optic layer.
The manufacture of a three-layer electro-optic display normally involves at least one lamination operation. For example, in several of the aforementioned MIT and E Ink patents and applications, there is described a process for manufacturing an encapsulated electrophoretic display in which an encapsulated electrophoretic medium comprising capsules in a binder is coated on to a flexible substrate comprising indium-tin-oxide or a similar conductive coating (which acts as an one electrode of the final display) on a plastic film, the capsules/binder coating being dried to form a coherent layer of the electrophoretic medium firmly adhered to the substrate. Separately, a backplane, containing an array of pixel electrodes and an appropriate arrangement of conductors to connect the pixel electrodes to drive circuitry, is prepared. To form the final display, the substrate having the capsule/binder layer thereon is laminated to the backplane using a lamination adhesive. (A very similar process can be used to prepare an electrophoretic display useable with a stylus or similar movable electrode by replacing the backplane with a simple protective layer, such as a plastic film, over which the stylus or other movable electrode can slide.) In one preferred form of such a process, the backplane is itself flexible and is prepared by printing the pixel electrodes and conductors on a plastic film or other flexible substrate. The obvious lamination technique for mass production of displays by this process is roll lamination using a lamination adhesive. Similar manufacturing techniques can be used with other types of electro-optic displays. For example, a microcell electrophoretic medium or a rotating bichromal member medium may be laminated to a backplane in substantially the same manner as an encapsulated electrophoretic medium.
In the processes described above, the lamination of the substrate carrying the electro-optic layer to the backplane may advantageously be carried out by vacuum lamination. Vacuum lamination is effective in expelling air from between the two materials being laminated, thus avoiding unwanted air bubbles in the final display; such air bubbles may introduce undesirable artifacts in the images produced on the display. (As discussed below, it may be desirable to produce the final lamination adhesive by blending multiple components. If this is done, it may be advantageous to allow the blended mixture to stand for some time before use to allow bubbles produced during blending to disperse.) However, vacuum lamination of the two parts of an electro-optic display in this manner imposes stringent requirements upon the lamination adhesive used, especially in the case of a display using an encapsulated electrophoretic medium. The lamination adhesive must have sufficient adhesive strength to bind the electro-optic layer to the layer (typically an electrode layer) to which it is to be laminated. The lamination adhesive must have adequate flow properties at the lamination temperature to ensure high quality lamination, and in this regard, the demands of laminating encapsulated electrophoretic and some other types of electro-optic media are unusually difficult; the lamination has be conducted at a temperature of not more than about 110xc2x0 C. since the medium cannot be exposed to substantially higher temperatures without damage, but the flow of the adhesive must cope with the relatively uneven surface of the capsule-containing layer, the surface of which is rendered irregular by the underlying capsules. The lamination temperature should indeed be kept as low as possible, and room temperature lamination would be ideal, but no commercial adhesive has been found which permits such room temperature lamination. The lamination adhesive must be chemically compatible with all the other materials in the display. Solvent-based lamination adhesives should be avoided; it has been found (although this does not appear to have been described in the literature), that any solvent left behind in the adhesive after lamination has a strong tendency to introduce undesirable contaminants into the electro-optic medium.
It has also been found that a lamination adhesive used in an electro-optic display must meet a variety of electrical criteria, and this introduces considerable problems in the selection of the lamination adhesive. Commercial manufacturers of lamination adhesives naturally devote considerable effort to ensuring that properties, such as strength of adhesion and lamination temperatures, of such adhesives are adjusted so that the adhesives perform well in their major applications, which typically involve laminating polymeric and similar films. However, in such applications, the electrical properties of the lamination adhesive are not relevant, and consequently the commercial manufacturers pay no heed to such electrical properties. Indeed, the present inventors have observed substantial variations (of up to several fold) in certain electrical properties between different batches of the same commercial lamination adhesive, presumably because the manufacturer was attempting to optimize non-electrical properties of the lamination adhesive (for example, resistance to bacterial growth) and was not at all concerned about resulting changes in electrical properties.
However, in electro-optic displays, in which the lamination adhesive is normally located between the electrodes which apply the electric field needed to change the electrical state of the electro-optic medium, the electrical properties of the adhesive become crucial. As will be apparent to electrical engineers, the volume resistivity of the lamination adhesive becomes important, since the voltage drop across the electro-optic medium is essentially equal to the voltage drop across the electrodes, minus the voltage drop across the lamination adhesive. If the resistivity of the adhesive layer is too high, a substantial voltage drop will occur within the adhesive layer, requiring an increase in voltage across the electrodes. Increasing the voltage across the electrodes in this manner is undesirable, since it increases the power consumption of the display, and may require the use of more complex and expensive control circuitry to handle the increased voltage involved. On the other hand, if the adhesive layer, which extends continuously across the display, is in contact with a matrix of electrodes, as in an active matrix display, the volume resistivity of the adhesive layer should not be too low, or lateral conduction of electric current through the continuous adhesive layer may cause undesirable cross-talk between adjacent electrodes. Also, since the volume resistivity of most materials decreases rapidly with increasing temperature, if the volume resistivity of the adhesive layer is too low, the performance of the display at temperatures substantially above room temperature is adversely affected. For these reasons, there is an optimum range of lamination adhesive resistivity values for use with any given electro-optic medium, this range varying with the resistivity of the electro-optic medium. The volume resistivities of encapsulated electrophoretic media are typically around 1010 ohm cm, and the resistivities of other electro-optic medium are usually of the same order of magnitude. Accordingly, the volume resistivity of the lamination adhesive should normally be around 108 to 1012 ohm cm, and preferably about 109 to 1011 ohm cm, at the operating temperature of the display, typically around 20xc2x0 C.
While it may be apparent that there should be a relationship between the volume resistivities of the electro-optic medium and the lamination adhesive used in an electro-optic display, the present inventors have discovered that other problems which have been observed in the operation of electro-optic displays, but which have not previously been understood, are attributable to the electrical and related properties of the lamination adhesive. For example, although the number of commercial materials which can meet most of the previously discussed, rather disparate requirements for a lamination adhesive for use in an electro-optic display is small, in practice it has been found that a small number of water-dispersed urethane emulsions, primarily polyester-based urethane emulsions, do appear to have most of the requisite properties. However, although these materials perform well when the displays are first produced, after the resultant displays have been operated for substantial periods of time (of the order of hundreds of hours) at room temperature, or stored for a similar period, the performance of the display suffers substantial degradation. This degradation first manifests itself as reduced white state reflectivity and slower or incomplete switching of the electro-optic medium, especially in areas where the lamination adhesive is thickest; the thickness of the lamination adhesive may vary across the display both because of a non-planar electro-optic layer, as for example in an encapsulated electrophoretic medium where the spherical or ellipsoidal capsules introduce deviations from planarity, and/or because the manufacturing process normally used to produce the electrode matrix in such displays produces a non-planar surface on the electrode matrix. This degradation increases at lower temperatures (10xc2x0 C. or below) and with time, so that after long periods the switching of the whole display is affected at room temperature. This degradation in optical performance with time is an important factor in limiting the service life of the displays.
The present inventors have discovered that the aforementioned degradation in performance is caused, at least in part, by changes in the volume resistivity of the lamination adhesive, and that this performance degradation of electrophoretic displays can be reduced or eliminated, and the service life of such displays increased, by using an adhesive the resistivity of which does not vary greatly with time; it appears that similar effects are produced in other types of electro-optic displays. The use of such an adhesive has also been found to improve the performance of the displays at low temperature, as manifested by improved reflectance in the light optical state of the display.
Accordingly, in one aspect the present invention seeks to provide electro-optic displays having optical characteristics which do not change rapidly with time, so that the displays have an improved operating lifetime.
Other problems known to occur in electro-optic displays, but which have not previously been explained, include degradation of the performance of the display with increasing temperature, even when the display is first produced, as manifested, inter alia, by a reduction in the contrast ratio of the display (the relative reflectance or optical transmission of the two extreme optical states of the display) with increasing temperature, the similar degradation of the performance of the display with increasing humidity, and the phenomenon known as xe2x80x9cself-erasingxe2x80x9d. See, for example, Ota, I., et al., xe2x80x9cDevelopments in Electrophoretic Displaysxe2x80x9d, Proceedings of the SID, 18, 243 (1977), where self-erasing was reported in an unencapsulated electrophoretic display. When the voltage applied across certain electrophoretic displays is switched off, the electrophoretic medium may reverse its optical state, and in some cases a reverse voltage, which may be larger than the operating voltage, can be observed to occur across the electrodes. It appears (although this invention is in no way limited by this belief) that the self-erasing phenomenon is due to a mismatch in electrical properties between various components of the display. Obviously, self-erasing is highly undesirable in that it reverses (or otherwise distorts, in the case of a grayscale display) the desired optical state of the display. It has been found that all of these problems may be attributable, at least in part, to changes in the electrical properties of the lamination adhesives with various environmental conditions, and that all can be reduced or eliminated by careful selection of the properties of the lamination adhesives used.
Accordingly, the present invention also seeks to provide lamination adhesives that can be used in the lamination of electro-optic displays at relatively low temperatures which do not adversely affect the electro-optic medium.
The present invention also seeks to provide an electro-optic display with a lamination adhesive having optimal mechanical properties.
The present invention also seeks to provide an electro-optic display with a lamination adhesive having optimal electrical properties.
In summary, the present invention seeks to provide a lamination adhesive with combined manufacturing, mechanical, electrical, environmental, chemical and temporal stability properties optimally suited for use in electro-optic displays.
The present invention also seeks to provide a novel polyurethane composition having properties which render it very suitable for use as a lamination adhesive in electro-optic displays.
Accordingly, in one aspect this invention provides an electro-optic display comprising first and second substrates and a lamination adhesive layer and a layer of a solid electro-optic material disposed between the first and second substrates. The lamination adhesive layer has a volume resistivity, measured at 10xc2x0 C., which does not change by a factor of more than about 3 after being held at 25xc2x0 C. and 45 percent relative humidity for 1000 hours. This form of the invention may hereinafter for convenience be referred to as the xe2x80x9cresistivity stabilityxe2x80x9d invention.
This invention also provides a process for preparing an electro-optic display. In this process, there is provided a first subassembly comprising an electro-optic layer and a first substrate, and a second subassembly comprising a second substrate, at least one of the subassemblies comprising an electrode. The two subassemblies are laminated to one another using a lamination adhesive so that the electro-optic layer is disposed between the first and second substrates, the lamination adhesive having a volume resistivity, measured at 10xc2x0 C., which does not change by a factor of more than about 3 after being held at 25xc2x0 C. and 45 percent relative humidity for 1000 hours.
This invention also provides an electro-optic display comprising first and second substrates, and a lamination adhesive layer and a layer of solid electro-optic material disposed between the first and second substrates. The lamination adhesive has any one or more of the following properties:
(a) having a volume resistivity, measured at 10xc2x0 C., which does not change by a factor of more than about 3 after being at 25xc2x0 C. and 45 percent relative humidity for 1000 hours;
(b) having a peel strength from an electrode material in contact with the lamination adhesive of at least about 2 lb/inch;
(c) the volume resistivity of the lamination adhesive changes by a factor of less than about 10 within a range of 10 to 90 percent relative humidity and over a temperature range of 10 to 50xc2x0 C.;
(d) the lamination adhesive has a thickness in the range of about 10 to about 20 xcexcm;
(e) the lamination adhesive has a shear modulus at 120xc2x0 C. of not more than about 1 megaPascal;
(f) the product of the dielectric constant and the volume resistivity of the lamination adhesive is not greater than the product of the dielectric constant and the volume resistivity of the electro-optic medium within a range of 10 to 90 percent relative humidity and over a temperature range of 10 to 50xc2x0 C.;
(g) comprising an ultra-violet stabilizer;
(h) comprising a light absorbing material.
This invention also provides a process for preparing an electro-optic display. In this process, there is provided a first subassembly comprising an electro-optic layer and a first substrate, and a second subassembly comprising a second substrate, at least one of the subassemblies comprising an electrode. The two subassemblies are laminated to one another using a lamination adhesive so that the electro-optic layer is disposed between the first and second substrates. The lamination adhesive has any one or more of the following properties:
(a) having a volume resistivity, measured at 10xc2x0 C., which does not change by a factor of more than about 3 after being held at 25xc2x0 C. and 45 percent relative humidity for 1000 hours;
(b) having a peel strength from an electrode material in contact with the lamination adhesive of at least about 2 lb/inch;
(c) the volume resistivity of the lamination adhesive changes by a factor of less than about 10 within a range of 10 to 90 percent relative humidity and over a temperature range of 10 to 50xc2x0 C.;
(d) the lamination adhesive has a thickness in the range of about 10 to about 20 xcexcm;
(e) the lamination adhesive has a shear modulus at 120xc2x0 C. of not more than about 1 megapascal;
(f) the product of the dielectric constant and the volume resistivity of the lamination adhesive is not greater than the product of the dielectric constant and the volume resistivity of the electro-optic medium within a range of 10 to 90 percent relative humidity and over a temperature range of 10 to 50xc2x0 C.;
(g) comprising an ultra-violet stabilizer;
(h) comprising a light absorbing material.
This invention also provides an electrophoretic display comprising first and second substrates, and a lamination adhesive layer and a layer of electrophoretic material disposed between the first and second substrates. The electrophoretic material comprises a plurality of capsules, each capsule comprising a capsule wall and an internal phase encapsulated within the capsule wall, the internal phase comprising electrically charged particles suspended in a suspending fluid and capable of moving through the fluid on application of an electric field to the electrophoretic material. The lamination adhesive has any one or more of the following properties:
(i) the product of the dielectric constant and the volume resistivity of the lamination adhesive is from about 0.01 to about 100 times the product of the dielectric constant and the volume resistivity of the suspending fluid;
(j) the ratio of the dielectric constant of the lamination adhesive to the dielectric constant of the suspending fluid within the temperature range of from 10 to 50xc2x0 C. does not vary from this ratio at 25xc2x0 C. by more than about 2 percent;
(k) the ratio of the volume resistivity of the lamination adhesive to the volume resistivity of the suspending fluid within the temperature range of from 10 to 50xc2x0 C. does not vary from this ratio at 25xc2x0 C. by more than a factor of about 100;
(l) the solubility of the suspending fluid in the lamination adhesive does not exceed about 1 percent weight/weight over the range of 10 to 50xc2x0 C.;
(m) being substantially free from mobile species.
This invention also provides a process for preparing an electrophoretic display. In this process there is provided a first subassembly comprising a first substrate and a layer of an electrophoretic medium comprising a plurality of capsules, each capsule comprising a capsule wall and an internal phase encapsulated within the capsule wall, the internal phase comprising electrically charged particles suspended in a suspending fluid and capable of moving through the fluid on application of an electric field to the electrophoretic medium, There is also provided a second subassembly comprising a second substrate; at least one of the subassemblies comprises an electrode. The two subassemblies are laminated to one another with a lamination adhesive so that the electro-optic layer is disposed between the first and second substrates. The lamination adhesive has any one or more of the following properties:
(i) the product of the dielectric constant and the volume resistivity of the lamination adhesive is from about 0.01 to about 100 times the product of the dielectric constant and the volume resistivity of the suspending fluid;
(j) the ratio of the dielectric constant of the lamination adhesive to the dielectric constant of the suspending fluid within the temperature range of from 10 to 50xc2x0 C. does not vary from this ratio at 25xc2x0 C. by more than about 2 percent;
(k) the ratio of the volume resistivity of the lamination adhesive to the volume resistivity of the suspending fluid within the temperature range of from 10 to 50xc2x0 C. does not vary from this ratio at 25xc2x0 C. by more than a factor of about 100;
(l) the solubility of the suspending fluid in the lamination adhesive does not exceed about 1 percent weight/weight over the range of 10 to 50xc2x0 C.;
(m) being substantially free from mobile species.
Finally, this invention provides a microcell electrophoretic display comprising a substrate having a plurality of closed cavities formed therein, said cavities being at least partially filled with a electrophoretic medium comprising a plurality of electrically charged particles suspended in a suspending fluid and capable of moving therethrough on application of an electric field to the electrophoretic medium, the microcell electrophoretic display further comprising at least one electrode and a layer of lamination adhesive disposed between the cavities and the electrode, the lamination adhesive being characterized by any one or more of the following:
(a) having a volume resistivity, measured at 10xc2x0 C., which does not change by a factor of more than about 3 after being held at 25xc2x0 C. and 45 percent relative humidity for 1000 hours;
(b) having a peel strength from an electrode material in contact with the lamination adhesive of at least about 2 lb/inch;
(c) the volume resistivity of the lamination adhesive changes by a factor of less than about 10 within a range of 10 to 90 percent relative humidity and over a temperature range of 10 to 50xc2x0 C.;
(d) the lamination adhesive has a thickness in the range of about 10 to about 20 xcexcm;
(e) the lamination adhesive has a shear modulus at 120xc2x0 C. of not more than about 1 megaPascal;
(f) the product of the dielectric constant and the volume resistivity of the lamination adhesive is not greater than the product of the dielectric constant and the volume resistivity of the electro-optic medium within a range of 10 to 90 percent relative humidity and over a temperature range of 10 to 50xc2x0 C.;
(g) comprising an ultra-violet stabilizer;
(h) comprising a light absorbing material;
(i) the product of the dielectric constant and the volume resistivity of the lamination adhesive is from about 0.01 to about 100 times the product of the dielectric constant and the volume resistivity of the suspending fluid;
(j) the ratio of the dielectric constant of the lamination adhesive to the dielectric constant of the suspending fluid within the temperature over the range of from 10 to 50xc2x0 C. does not vary from this ratio at 25xc2x0 C. by more than about 2 percent;
(k) the ratio of the volume resistivity of the lamination adhesive to the volume resistivity of the suspending fluid within the temperature over the range of from 10 to 50xc2x0 C. does not vary from this ratio at 25xc2x0 C. by more than a factor of about 100;
(l) the solubility of the suspending fluid in the lamination adhesive does not exceed about 1 percent weight/weight over the range of 10 to 50xc2x0 C.;
(m) being substantially free from mobile species.